Free Lossless Audio Codec
draft-ietf-cellar-flac-01
The information below is for an old version of the document.
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| Authors | Michael Richardson , Andrew Weaver | ||
| Last updated | 2021-04-27 | ||
| Replaces | draft-weaver-cellar-flac | ||
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draft-ietf-cellar-flac-01
cellar M. Sandelman
Internet-Draft
Intended status: Informational A. Weaver
Expires: 29 October 2021 27 April 2021
Free Lossless Audio Codec
draft-ietf-cellar-flac-01
Abstract
This document defines FLAC, which stands for Free Lossless Audio
Codec, a free, open source codec for lossless audio compression and
decompression.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 29 October 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notation and Conventions . . . . . . . . . . . . . . . . . . 3
3. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 3
4. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Blocking . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Interchannel Decorrelation . . . . . . . . . . . . . . . . . 6
9. Prediction . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Residual Coding . . . . . . . . . . . . . . . . . . . . . . . 8
11. Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Conventions . . . . . . . . . . . . . . . . . . . . . . 12
11.2. STREAM . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.3. METADATA_BLOCK . . . . . . . . . . . . . . . . . . . . . 13
11.4. METADATA_BLOCK_HEADER . . . . . . . . . . . . . . . . . 13
11.5. BLOCK_TYPE . . . . . . . . . . . . . . . . . . . . . . . 14
11.6. METADATA_BLOCK_DATA . . . . . . . . . . . . . . . . . . 14
11.7. METADATA_BLOCK_STREAMINFO . . . . . . . . . . . . . . . 15
11.8. METADATA_BLOCK_PADDING . . . . . . . . . . . . . . . . . 16
11.9. METADATA_BLOCK_APPLICATION . . . . . . . . . . . . . . . 16
11.10. METADATA_BLOCK_SEEKTABLE . . . . . . . . . . . . . . . . 16
11.11. SEEKPOINT . . . . . . . . . . . . . . . . . . . . . . . 17
11.12. METADATA_BLOCK_VORBIS_COMMENT . . . . . . . . . . . . . 17
11.13. METADATA_BLOCK_CUESHEET . . . . . . . . . . . . . . . . 18
11.14. CUESHEET_TRACK . . . . . . . . . . . . . . . . . . . . . 19
11.15. CUESHEET_TRACK_INDEX . . . . . . . . . . . . . . . . . . 20
11.16. METADATA_BLOCK_PICTURE . . . . . . . . . . . . . . . . . 20
11.17. PICTURE_TYPE . . . . . . . . . . . . . . . . . . . . . . 21
11.18. FRAME . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.19. FRAME_HEADER . . . . . . . . . . . . . . . . . . . . . . 23
11.19.1. FRAME HEADER RESERVED . . . . . . . . . . . . . . . 23
11.19.2. BLOCKING STRATEGY . . . . . . . . . . . . . . . . . 24
11.19.3. INTERCHANNEL SAMPLE BLOCK SIZE . . . . . . . . . . 24
11.19.4. SAMPLE RATE . . . . . . . . . . . . . . . . . . . . 25
11.19.5. CHANNEL ASSIGNMENT . . . . . . . . . . . . . . . . 26
11.19.6. SAMPLE SIZE . . . . . . . . . . . . . . . . . . . . 27
11.19.7. FRAME HEADER RESERVED2 . . . . . . . . . . . . . . 27
11.19.8. CODED NUMBER . . . . . . . . . . . . . . . . . . . 27
11.19.9. BLOCK SIZE INT . . . . . . . . . . . . . . . . . . 28
11.19.10. SAMPLE RATE INT . . . . . . . . . . . . . . . . . . 28
11.19.11. FRAME CRC . . . . . . . . . . . . . . . . . . . . . 28
11.20. FRAME_FOOTER . . . . . . . . . . . . . . . . . . . . . . 28
11.21. SUBFRAME . . . . . . . . . . . . . . . . . . . . . . . . 29
11.22. SUBFRAME_HEADER . . . . . . . . . . . . . . . . . . . . 29
11.22.1. SUBFRAME TYPE . . . . . . . . . . . . . . . . . . . 29
11.22.2. WASTED BITS PER SAMPLE FLAG . . . . . . . . . . . . 30
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11.23. SUBFRAME_CONSTANT . . . . . . . . . . . . . . . . . . . 30
11.24. SUBFRAME_FIXED . . . . . . . . . . . . . . . . . . . . . 30
11.25. SUBFRAME_LPC . . . . . . . . . . . . . . . . . . . . . . 31
11.26. SUBFRAME_VERBATIM . . . . . . . . . . . . . . . . . . . 31
11.27. RESIDUAL . . . . . . . . . . . . . . . . . . . . . . . . 31
11.27.1. RESIDUAL_CODING_METHOD . . . . . . . . . . . . . . 32
11.27.2. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB . . . 32
11.27.3. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2 . . 33
11.27.4. ENCODED RESIDUAL . . . . . . . . . . . . . . . . . 34
12. Normative References . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
This is a detailed description of the FLAC format. There is also a
companion document that describes FLAC-to-Ogg mapping
(https://xiph.org/flac/ogg_mapping.html).
For a user-oriented overview, see About the FLAC Format
(https://xiph.org/flac/documentation_format_overview.html).
2. Notation and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Acknowledgments
FLAC owes much to the many people who have advanced the audio
compression field so freely. For instance: - A. J. Robinson
(http://svr-www.eng.cam.ac.uk/~ajr/) for his work on Shorten
(http://svr-www.eng.cam.ac.uk/reports/abstracts/robinson_tr156.html);
his paper is a good starting point on some of the basic methods used
by FLAC. FLAC trivially extends and improves the fixed predictors,
LPC coefficient quantization, and Exponential-Golomb coding used in
Shorten. - S. W. Golomb
(https://web.archive.org/web/20040215005354/http://csi.usc.edu/
faculty/golomb.html) and Robert F. Rice; their universal codes are
used by FLAC's entropy coder. - N. Levinson and J. Durbin; the
reference encoder uses an algorithm developed and refined by them for
determining the LPC coefficients from the autocorrelation
coefficients. - And of course, Claude Shannon
(http://en.wikipedia.org/wiki/Claude_Shannon)
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4. Scope
It is a known fact that no algorithm can losslessly compress all
possible input, so most compressors restrict themselves to a useful
domain and try to work as well as possible within that domain.
FLAC's domain is audio data. Though it can losslessly code any
input, only certain kinds of input will get smaller. FLAC exploits
the fact that audio data typically has a high degree of sample-to-
sample correlation.
Within the audio domain, there are many possible subdomains. For
example: low bitrate speech, high-bitrate multi-channel music, etc.
FLAC itself does not target a specific subdomain, but many of the
default parameters of the reference encoder are tuned to CD-quality
music data (i.e. 44.1 kHz, 2 channel, 16 bits per sample). The
effect of the encoding parameters on different kinds of audio data
will be examined later.
5. Architecture
Similar to many audio coders, a FLAC encoder has the following
stages:
* "Blocking" (see section on Blocking (#blocking)). The input is
broken up into many contiguous blocks. With FLAC, the blocks MAY
vary in size. The optimal size of the block is usually affected
by many factors, including the sample rate, spectral
characteristics over time, etc. Though FLAC allows the block size
to vary within a stream, the reference encoder uses a fixed block
size.
* "Interchannel Decorrelation" (see section on Interchannel
Decorrelation (#interchannel-decorrelation)). In the case of
stereo streams, the encoder will create mid and side signals based
on the average and difference (respectively) of the left and right
channels. The encoder will then pass the best form of the signal
to the next stage.
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* "Prediction" (see section on Prediction (#prediction)). The block
is passed through a prediction stage where the encoder tries to
find a mathematical description (usually an approximate one) of
the signal. This description is typically much smaller than the
raw signal itself. Since the methods of prediction are known to
both the encoder and decoder, only the parameters of the predictor
need be included in the compressed stream. FLAC currently uses
four different classes of predictors, but the format has reserved
space for additional methods. FLAC allows the class of predictor
to change from block to block, or even within the channels of a
block.
* "Residual Coding" (See section on Residual Coding (#residual-
coding)). If the predictor does not describe the signal exactly,
the difference between the original signal and the predicted
signal (called the error or residual signal) MUST be coded
losslessly. If the predictor is effective, the residual signal
will require fewer bits per sample than the original signal. FLAC
currently uses only one method for encoding the residual, but the
format has reserved space for additional methods. FLAC allows the
residual coding method to change from block to block, or even
within the channels of a block.
In addition, FLAC specifies a metadata system, which allows arbitrary
information about the stream to be included at the beginning of the
stream.
6. Definitions
Many terms like "block" and "frame" are used to mean different things
in different encoding schemes. For example, a frame in MP3
corresponds to many samples across several channels, whereas an S/
PDIF frame represents just one sample for each channel. The
definitions we use for FLAC follow. Note that when we talk about
blocks and subblocks we are referring to the raw unencoded audio data
that is the input to the encoder, and when we talk about frames and
subframes, we are referring to the FLAC-encoded data.
* *Block*: One or more audio samples that span several channels.
* *Subblock*: One or more audio samples within a channel. A block
contains one subblock for each channel, and all subblocks contain
the same number of samples.
* *Blocksize*: The number of samples in any of a block's subblocks.
For example, a one second block sampled at 44.1 kHz has a
blocksize of 44100, regardless of the number of channels.
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* *Frame*: A frame header plus one or more subframes.
* *Subframe*: A subframe header plus one or more encoded samples
from a given channel. All subframes within a frame will contain
the same number of samples.
* *Exponential-Golomb coding*: One of Robert Rice's universal coding
schemes, FLAC's residual coder, compresses data by writing the
number of bits to be read minus 1, before writing the actual
value.
* *LPC*: Linear predictive coding (https://en.wikipedia.org/wiki/
Linear_predictive_coding).
7. Blocking
The size used for blocking the audio data has a direct effect on the
compression ratio. If the block size is too small, the resulting
large number of frames mean that excess bits will be wasted on frame
headers. If the block size is too large, the characteristics of the
signal MAY vary so much that the encoder will be unable to find a
good predictor. In order to simplify encoder/decoder design, FLAC
imposes a minimum block size of 16 samples, and a maximum block size
of 65535 samples. This range covers the optimal size for all of the
audio data FLAC supports.
Currently the reference encoder uses a fixed block size, optimized on
the sample rate of the input. Future versions MAY vary the block
size depending on the characteristics of the signal.
Blocked data is passed to the predictor stage one subblock (channel)
at a time. Each subblock is independently coded into a subframe, and
the subframes are concatenated into a frame. Because each channel is
coded separately, one channel of a stereo frame MAY be encoded as a
constant subframe, and the other an LPC subframe.
8. Interchannel Decorrelation
In stereo streams, many times there is an exploitable amount of
correlation between the left and right channels. FLAC allows the
frames of stereo streams to have different channel assignments, and
an encoder MAY choose to use the best representation on a frame-by-
frame basis.
* *Independent*. The left and right channels are coded
independently.
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* *Mid-side*. The left and right channels are transformed into mid
and side channels. The mid channel is the midpoint (average) of
the left and right signals, and the side is the difference signal
(left minus right).
* *Left-side*. The left channel and side channel are coded.
* *Right-side*. The right channel and side channel are coded.
Surprisingly, the left-side and right-side forms can be the most
efficient in many frames, even though the raw number of bits per
sample needed for the original signal is slightly more than that
needed for independent or mid-side coding.
9. Prediction
FLAC uses four methods for modeling the input signal:
1. *Verbatim*. This is essentially a zero-order predictor of the
signal. The predicted signal is zero, meaning the residual is
the signal itself, and the compression is zero. This is the
baseline against which the other predictors are measured. If you
feed random data to the encoder, the verbatim predictor will
probably be used for every subblock. Since the raw signal is not
actually passed through the residual coding stage (it is added to
the stream 'verbatim'), the encoding results will not be the same
as a zero-order linear predictor.
2. *Constant*. This predictor is used whenever the subblock is pure
DC ("digital silence"), i.e. a constant value throughout. The
signal is run-length encoded and added to the stream.
3. *Fixed linear predictor*. FLAC uses a class of computationally-
efficient fixed linear predictors (for a good description, see
audiopak (http://www.hpl.hp.com/techreports/1999/HPL-
1999-144.pdf) and shorten (http://svr-
www.eng.cam.ac.uk/reports/abstracts/robinson_tr156.html)). FLAC
adds a fourth-order predictor to the zero-to-third-order
predictors used by Shorten. Since the predictors are fixed, the
predictor order is the only parameter that needs to be stored in
the compressed stream. The error signal is then passed to the
residual coder.
4. *FIR Linear prediction*. For more accurate modeling (at a cost of
slower encoding), FLAC supports up to 32nd order FIR linear
prediction (again, for information on linear prediction, see
audiopak (http://www.hpl.hp.com/techreports/1999/HPL-
1999-144.pdf) and shorten (http://svr-
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www.eng.cam.ac.uk/reports/abstracts/robinson_tr156.html)). The
reference encoder uses the Levinson-Durbin method for calculating
the LPC coefficients from the autocorrelation coefficients, and
the coefficients are quantized before computing the residual.
Whereas encoders such as Shorten used a fixed quantization for
the entire input, FLAC allows the quantized coefficient precision
to vary from subframe to subframe. The FLAC reference encoder
estimates the optimal precision to use based on the block size
and dynamic range of the original signal.
10. Residual Coding
FLAC uses Exponential-Golomb (a variant of Rice) coding as its
residual encoder. You can learn more about exp-golomb coding
(https://en.wikipedia.org/wiki/Exponential-Golomb_coding) on
Wikipedia.
FLAC currently defines two similar methods for the coding of the
error signal from the prediction stage. The error signal is coded
using Exponential-Golomb codes in one of two ways:
1. the encoder estimates a single exp-golomb parameter based on the
variance of the residual and exp-golomb codes the entire residual
using this parameter;
2. the residual is partitioned into several equal-length regions of
contiguous samples, and each region is coded with its own exp-
golomb parameter based on the region's mean.
(Note that the first method is a special case of the second method
with one partition, except the exp-golomb parameter is based on the
residual variance instead of the mean.)
The FLAC format has reserved space for other coding methods. Some
possibilities for volunteers would be to explore better context-
modeling of the exp-golomb parameter, or Huffman coding. See LOCO-I
(http://www.hpl.hp.com/techreports/98/HPL-98-193.html) and pucrunch (
http://web.archive.org/web/20140827133312/http://www.cs.tut.fi/~alber
t/Dev/pucrunch/packing.html) for descriptions of several universal
codes.
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11. Format
This section specifies the FLAC bitstream format. FLAC has no format
version information, but it does contain reserved space in several
places. Future versions of the format MAY use this reserved space
safely without breaking the format of older streams. Older decoders
MAY choose to abort decoding or skip data encoded with newer methods.
Apart from reserved patterns, in places the format specifies invalid
patterns, meaning that the patterns MAY never appear in any valid
bitstream, in any prior, present, or future versions of the format.
These invalid patterns are usually used to make the synchronization
mechanism more robust.
All numbers used in a FLAC bitstream MUST be integers; there are no
floating-point representations. All numbers MUST be big-endian
coded. All numbers MUST be unsigned unless otherwise specified.
Before the formal description of the stream, an overview might be
helpful.
* A FLAC bitstream consists of the "fLaC" (i.e. 0x664C6143) marker
at the beginning of the stream, followed by a mandatory metadata
block (called the STREAMINFO block), any number of other metadata
blocks, then the audio frames.
* FLAC supports up to 128 kinds of metadata blocks; currently the
following are defined:
- "STREAMINFO": This block has information about the whole
stream, like sample rate, number of channels, total number of
samples, etc. It MUST be present as the first metadata block
in the stream. Other metadata blocks MAY follow, and ones that
the decoder doesn't understand, it will skip.
- "PADDING": This block allows for an arbitrary amount of
padding. The contents of a PADDING block have no meaning.
This block is useful when it is known that metadata will be
edited after encoding; the user can instruct the encoder to
reserve a PADDING block of sufficient size so that when
metadata is added, it will simply overwrite the padding (which
is relatively quick) instead of having to insert it into the
right place in the existing file (which would normally require
rewriting the entire file).
- "APPLICATION": This block is for use by third-party
applications. The only mandatory field is a 32-bit identifier.
This ID is granted upon request to an application by the FLAC
maintainers. The remainder is of the block is defined by the
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registered application. Visit the registration page
(https://xiph.org/flac/id.html) if you would like to register
an ID for your application with FLAC.
- "SEEKTABLE": This is an OPTIONAL block for storing seek points.
It is possible to seek to any given sample in a FLAC stream
without a seek table, but the delay can be unpredictable since
the bitrate MAY vary widely within a stream. By adding seek
points to a stream, this delay can be significantly reduced.
Each seek point takes 18 bytes, so 1% resolution within a
stream adds less than 2K. There can be only one SEEKTABLE in a
stream, but the table can have any number of seek points.
There is also a special 'placeholder' seekpoint which will be
ignored by decoders but which can be used to reserve space for
future seek point insertion.
- "VORBIS_COMMENT": This block is for storing a list of human-
readable name/value pairs. Values are encoded using UTF-8. It
is an implementation of the Vorbis comment specification
(http://xiph.org/vorbis/doc/v-comment.html) (without the
framing bit). This is the only officially supported tagging
mechanism in FLAC. There MUST be only zero or one
VORBIS_COMMENT blocks in a stream. In some external
documentation, Vorbis comments are called FLAC tags to lessen
confusion.
- "CUESHEET": This block is for storing various information that
can be used in a cue sheet. It supports track and index
points, compatible with Red Book CD digital audio discs, as
well as other CD-DA metadata such as media catalog number and
track ISRCs. The CUESHEET block is especially useful for
backing up CD-DA discs, but it can be used as a general purpose
cueing mechanism for playback.
- "PICTURE": This block is for storing pictures associated with
the file, most commonly cover art from CDs. There MAY be more
than one PICTURE block in a file. The picture format is
similar to the APIC frame in ID3v2 (http://www.id3.org/
id3v2.4.0-frames). The PICTURE block has a type, MIME type,
and UTF-8 description like ID3v2, and supports external linking
via URL (though this is discouraged). The differences are that
there is no uniqueness constraint on the description field, and
the MIME type is mandatory. The FLAC PICTURE block also
includes the resolution, color depth, and palette size so that
the client can search for a suitable picture without having to
scan them all.
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* The audio data is composed of one or more audio frames. Each
frame consists of a frame header, which contains a sync code,
information about the frame like the block size, sample rate,
number of channels, et cetera, and an 8-bit CRC. The frame header
also contains either the sample number of the first sample in the
frame (for variable-blocksize streams), or the frame number (for
fixed-blocksize streams). This allows for fast, sample-accurate
seeking to be performed. Following the frame header are encoded
subframes, one for each channel, and finally, the frame is zero-
padded to a byte boundary. Each subframe has its own header that
specifies how the subframe is encoded.
* Since a decoder MAY start decoding in the middle of a stream,
there MUST be a method to determine the start of a frame. A
14-bit sync code begins each frame. The sync code will not appear
anywhere else in the frame header. However, since it MAY appear
in the subframes, the decoder has two other ways of ensuring a
correct sync. The first is to check that the rest of the frame
header contains no invalid data. Even this is not foolproof since
valid header patterns can still occur within the subframes. The
decoder's final check is to generate an 8-bit CRC of the frame
header and compare this to the CRC stored at the end of the frame
header.
* Again, since a decoder MAY start decoding at an arbitrary frame in
the stream, each frame header MUST contain some basic information
about the stream because the decoder MAY not have access to the
STREAMINFO metadata block at the start of the stream. This
information includes sample rate, bits per sample, number of
channels, etc. Since the frame header is pure overhead, it has a
direct effect on the compression ratio. To keep the frame header
as small as possible, FLAC uses lookup tables for the most
commonly used values for frame parameters. For instance, the
sample rate part of the frame header is specified using 4 bits.
Eight of the bit patterns correspond to the commonly used sample
rates of 8, 16, 22.05, 24, 32, 44.1, 48 or 96 kHz. However, odd
sample rates can be specified by using one of the 'hint' bit
patterns, directing the decoder to find the exact sample rate at
the end of the frame header. The same method is used for
specifying the block size and bits per sample. In this way, the
frame header size stays small for all of the most common forms of
audio data.
* Individual subframes (one for each channel) are coded separately
within a frame, and appear serially in the stream. In other
words, the encoded audio data is NOT channel-interleaved. This
reduces decoder complexity at the cost of requiring larger decode
buffers. Each subframe has its own header specifying the
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attributes of the subframe, like prediction method and order,
residual coding parameters, etc. The header is followed by the
encoded audio data for that channel.
* "FLAC" specifies a subset of itself as the Subset format. The
purpose of this is to ensure that any streams encoded according to
the Subset are truly "streamable", meaning that a decoder that
cannot seek within the stream can still pick up in the middle of
the stream and start decoding. It also makes hardware decoder
implementations more practical by limiting the encoding parameters
such that decoder buffer sizes and other resource requirements can
be easily determined. *flac* generates Subset streams by default
unless the "--lax" command-line option is used. The Subset makes
the following limitations on what MAY be used in the stream:
* The blocksize bits in the "FRAME_HEADER" (see FRAME_HEADER section
(#frameheader)) MUST be 0b0001-0b1110. The blocksize MUST be <=
16384; if the sample rate is <= 48000 Hz, the blocksize MUST be <=
4608 = 2^9 * 3^2.
* The sample rate bits in the "FRAME_HEADER" MUST be 0b0001-0b1110.
* The bits-per-sample bits in the "FRAME_HEADER" MUST be
0b001-0b111.
* If the sample rate is <= 48000 Hz, the filter order in "LPC
subframes" (see SUBFRAME_LPC section (#subframelpc)) MUST be less
than or equal to 12, i.e. the subframe type bits in the
"SUBFRAME_HEADER" (see SUBFRAME_HEADER section (#subframeheader))
SHOULD NOT be 0b101100-0b111111.
* The Rice partition order in an "exp-golomb coded residual section"
(see RESIDUAL_CODING_METHOD_PARTITIONE_EXP_GOLOMB section
(#residualcodingmethodpartitionedexpgolomb)) MUST be less than or
equal to 8.
11.1. Conventions
The following tables constitute a formal description of the FLAC
format. Values expressed as "u(n)" represent unsigned big-endian
integer using "n" bits. "n" may be expressed as an equation using "*"
(multiplication), "/" (division), "+" (addition), or "-"
(subtraction). An inclusive range of the number of bits expressed
may be represented with an ellipsis, such as "u(m...n)". The name of
a value followed by an asterisk "*" indicates zero or more
occurrences of the value. The name of a value followed by a plus
sign "+" indicates one or more occurrences of the value.
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11.2. STREAM
+===========================+=====================================+
| Data | Description |
+===========================+=====================================+
| u(32) | "fLaC", the FLAC stream marker in |
| | ASCII, meaning byte 0 of the stream |
| | is 0x66, followed by 0x4C 0x61 0x43 |
+---------------------------+-------------------------------------+
| METADATA_BLOCK_STREAMINFO | This is the mandatory STREAMINFO |
| | metadata block that has the basic |
| | properties of the stream. |
+---------------------------+-------------------------------------+
| "METADATA_BLOCK"* | Zero or more metadata blocks |
+---------------------------+-------------------------------------+
| "FRAME"+ | One or more audio frames |
+---------------------------+-------------------------------------+
Table 1
11.3. METADATA_BLOCK
+=======================+========================================+
| Data | Description |
+=======================+========================================+
| METADATA_BLOCK_HEADER | A block header that specifies the type |
| | and size of the metadata block data. |
+-----------------------+----------------------------------------+
| METADATA_BLOCK_DATA | |
+-----------------------+----------------------------------------+
Table 2
11.4. METADATA_BLOCK_HEADER
+=======+=========================================================+
| Data | Description |
+=======+=========================================================+
| u(1) | Last-metadata-block flag: '1' if this block is the last |
| | metadata block before the audio blocks, '0' otherwise. |
+-------+---------------------------------------------------------+
| u(7) | BLOCK_TYPE |
+-------+---------------------------------------------------------+
| u(24) | Length (in bytes) of metadata to follow (does not |
| | include the size of the "METADATA_BLOCK_HEADER") |
+-------+---------------------------------------------------------+
Table 3
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11.5. BLOCK_TYPE
+=========+====================================================+
| Value | Description |
+=========+====================================================+
| 0 | STREAMINFO |
+---------+----------------------------------------------------+
| 1 | PADDING |
+---------+----------------------------------------------------+
| 2 | APPLICATION |
+---------+----------------------------------------------------+
| 3 | SEEKTABLE |
+---------+----------------------------------------------------+
| 4 | VORBIS_COMMENT |
+---------+----------------------------------------------------+
| 5 | CUESHEET |
+---------+----------------------------------------------------+
| 6 | PICTURE |
+---------+----------------------------------------------------+
| 7 - 126 | reserved |
+---------+----------------------------------------------------+
| 127 | invalid, to avoid confusion with a frame sync code |
+---------+----------------------------------------------------+
Table 4
11.6. METADATA_BLOCK_DATA
+====================================+=============+
| Data | Description |
+====================================+=============+
| "METADATA_BLOCK_STREAMINFO" || | The block |
| "METADATA_BLOCK_PADDING" || | data MUST |
| "METADATA_BLOCK_APPLICATION" || | match the |
| "METADATA_BLOCK_SEEKTABLE" || | block type |
| "METADATA_BLOCK_VORBIS_COMMENT" || | in the |
| "METADATA_BLOCK_CUESHEET" || | block |
| "METADATA_BLOCK_PICTURE" | header. |
+------------------------------------+-------------+
Table 5
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11.7. METADATA_BLOCK_STREAMINFO
+========+=================================================+
| Data | Description |
+========+=================================================+
| u(16) | The minimum block size (in samples) used in the |
| | stream. |
+--------+-------------------------------------------------+
| u(16) | The maximum block size (in samples) used in the |
| | stream. (Minimum blocksize == maximum |
| | blocksize) implies a fixed-blocksize stream. |
+--------+-------------------------------------------------+
| u(24) | The minimum frame size (in bytes) used in the |
| | stream. A value of "0" signifies that the |
| | value is not known. |
+--------+-------------------------------------------------+
| u(24) | The maximum frame size (in bytes) used in the |
| | stream. A value of "0" signifies that the |
| | value is not known. |
+--------+-------------------------------------------------+
| u(20) | Sample rate in Hz. Though 20 bits are |
| | available, the maximum sample rate is limited |
| | by the structure of frame headers to 655350 Hz. |
| | Also, a value of 0 is invalid. |
+--------+-------------------------------------------------+
| u(3) | (number of channels)-1. FLAC supports from 1 |
| | to 8 channels |
+--------+-------------------------------------------------+
| u(5) | (bits per sample)-1. FLAC supports from 4 to |
| | 32 bits per sample. Currently the reference |
| | encoder and decoders only support up to 24 bits |
| | per sample. |
+--------+-------------------------------------------------+
| u(36) | Total samples in stream. 'Samples' means |
| | inter-channel sample, i.e. one second of 44.1 |
| | kHz audio will have 44100 samples regardless of |
| | the number of channels. A value of zero here |
| | means the number of total samples is unknown. |
+--------+-------------------------------------------------+
| u(128) | MD5 signature of the unencoded audio data. |
| | This allows the decoder to determine if an |
| | error exists in the audio data even when the |
| | error does not result in an invalid bitstream. |
+--------+-------------------------------------------------+
Table 6
NOTE
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* FLAC specifies a minimum block size of 16 and a maximum block size
of 65535, meaning the bit patterns corresponding to the numbers
0-15 in the minimum blocksize and maximum blocksize fields are
invalid.
11.8. METADATA_BLOCK_PADDING
+======+========================================+
| Data | Description |
+======+========================================+
| u(n) | n '0' bits (n MUST be a multiple of 8) |
+------+----------------------------------------+
Table 7
11.9. METADATA_BLOCK_APPLICATION
+=======+===========================================+
| Data | Description |
+=======+===========================================+
| u(32) | Registered application ID. (Visit the |
| | registration page (https://xiph.org/flac/ |
| | id.html) to register an ID with FLAC.) |
+-------+-------------------------------------------+
| u(n) | Application data (n MUST be a multiple of |
| | 8) |
+-------+-------------------------------------------+
Table 8
11.10. METADATA_BLOCK_SEEKTABLE
+==============+==========================+
| Data | Description |
+==============+==========================+
| "SEEKPOINT"+ | One or more seek points. |
+--------------+--------------------------+
Table 9
NOTE - The number of seek points is implied by the metadata header
'length' field, i.e. equal to length / 18.
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11.11. SEEKPOINT
+=======+==========================================================+
| Data | Description |
+=======+==========================================================+
| u(64) | Sample number of first sample in the target frame, or |
| | "0xFFFFFFFFFFFFFFFF" for a placeholder point. |
+-------+----------------------------------------------------------+
| u(64) | Offset (in bytes) from the first byte of the first frame |
| | header to the first byte of the target frame's header. |
+-------+----------------------------------------------------------+
| u(16) | Number of samples in the target frame. |
+-------+----------------------------------------------------------+
Table 10
NOTES
* For placeholder points, the second and third field values are
undefined.
* Seek points within a table MUST be sorted in ascending order by
sample number.
* Seek points within a table MUST be unique by sample number, with
the exception of placeholder points.
* The previous two notes imply that there MAY be any number of
placeholder points, but they MUST all occur at the end of the
table.
11.12. METADATA_BLOCK_VORBIS_COMMENT
+======+===========================================================+
| Data | Description |
+======+===========================================================+
| u(n) | Also known as FLAC tags, the contents of a vorbis comment |
| | packet as specified here (http://www.xiph.org/vorbis/doc/ |
| | v-comment.html) (without the framing bit). Note that the |
| | vorbis comment spec allows for on the order of 2^64 bytes |
| | of data where as the FLAC metadata block is limited to |
| | 2^24 bytes. Given the stated purpose of vorbis comments, |
| | i.e. human-readable textual information, this limit is |
| | unlikely to be restrictive. Also note that the 32-bit |
| | field lengths are little-endian coded according to the |
| | vorbis spec, as opposed to the usual big-endian coding of |
| | fixed-length integers in the rest of FLAC. |
+------+-----------------------------------------------------------+
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Table 11
11.13. METADATA_BLOCK_CUESHEET
+===================+==============================================+
| Data | Description |
+===================+==============================================+
| u(128*8) | Media catalog number, in ASCII printable |
| | characters 0x20-0x7E. In general, the media |
| | catalog number SHOULD be 0 to 128 bytes |
| | long; any unused characters SHOULD be right- |
| | padded with NUL characters. For CD-DA, this |
| | is a thirteen digit number, followed by 115 |
| | NUL bytes. |
+-------------------+----------------------------------------------+
| u(64) | The number of lead-in samples. This field |
| | has meaning only for CD-DA cuesheets; for |
| | other uses it SHOULD be 0. For CD-DA, the |
| | lead-in is the TRACK 00 area where the table |
| | of contents is stored; more precisely, it is |
| | the number of samples from the first sample |
| | of the media to the first sample of the |
| | first index point of the first track. |
| | According to the Red Book, the lead-in MUST |
| | be silence and CD grabbing software does not |
| | usually store it; additionally, the lead-in |
| | MUST be at least two seconds but MAY be |
| | longer. For these reasons the lead-in |
| | length is stored here so that the absolute |
| | position of the first track can be computed. |
| | Note that the lead-in stored here is the |
| | number of samples up to the first index |
| | point of the first track, not necessarily to |
| | INDEX 01 of the first track; even the first |
| | track MAY have INDEX 00 data. |
+-------------------+----------------------------------------------+
| u(1) | "1" if the CUESHEET corresponds to a Compact |
| | Disc, else "0". |
+-------------------+----------------------------------------------+
| u(7+258*8) | Reserved. All bits MUST be set to zero. |
+-------------------+----------------------------------------------+
| u(8) | The number of tracks. Must be at least 1 |
| | (because of the requisite lead-out track). |
| | For CD-DA, this number MUST be no more than |
| | 100 (99 regular tracks and one lead-out |
| | track). |
+-------------------+----------------------------------------------+
| "CUESHEET_TRACK"+ | One or more tracks. A CUESHEET block is |
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| | REQUIRED to have a lead-out track; it is |
| | always the last track in the CUESHEET. For |
| | CD-DA, the lead-out track number MUST be 170 |
| | as specified by the Red Book, otherwise it |
| | MUST be 255. |
+-------------------+----------------------------------------------+
Table 12
11.14. CUESHEET_TRACK
+=======================+=================================================+
|Data |Description |
+=======================+=================================================+
|u(64) |Track offset in samples, relative to the |
| |beginning of the FLAC audio stream. It is the |
| |offset to the first index point of the track. |
| |(Note how this differs from CD-DA, where the |
| |track's offset in the TOC is that of the track's |
| |INDEX 01 even if there is an INDEX 00.) For CD- |
| |DA, the offset MUST be evenly divisible by 588 |
| |samples (588 samples = 44100 samples/s * 1/75 s).|
+-----------------------+-------------------------------------------------+
|u(8) |Track number. A track number of 0 is not allowed|
| |to avoid conflicting with the CD-DA spec, which |
| |reserves this for the lead-in. For CD-DA the |
| |number MUST be 1-99, or 170 for the lead-out; for|
| |non-CD-DA, the track number MUST for 255 for the |
| |lead-out. It is not REQUIRED but encouraged to |
| |start with track 1 and increase sequentially. |
| |Track numbers MUST be unique within a CUESHEET. |
+-----------------------+-------------------------------------------------+
|u(12*8) |Track ISRC. This is a 12-digit alphanumeric |
| |code; see here (http://isrc.ifpi.org/) and here |
| |(http://www.disctronics.co.uk/technology/cdaudio/|
| |cdaud_isrc.htm). A value of 12 ASCII NUL |
| |characters MAY be used to denote absence of an |
| |ISRC. |
+-----------------------+-------------------------------------------------+
|u(1) |The track type: 0 for audio, 1 for non-audio. |
| |This corresponds to the CD-DA Q-channel control |
| |bit 3. |
+-----------------------+-------------------------------------------------+
|u(1) |The pre-emphasis flag: 0 for no pre-emphasis, 1 |
| |for pre-emphasis. This corresponds to the CD-DA |
| |Q-channel control bit 5; see here |
| |(http://www.chipchapin.com/CDMedia/cdda9.php3). |
+-----------------------+-------------------------------------------------+
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|u(6+13*8) |Reserved. All bits MUST be set to zero. |
+-----------------------+-------------------------------------------------+
|u(8) |The number of track index points. There MUST be |
| |at least one index in every track in a CUESHEET |
| |except for the lead-out track, which MUST have |
| |zero. For CD-DA, this number SHOULD NOT be more |
| |than 100. |
+-----------------------+-------------------------------------------------+
|"CUESHEET_TRACK_INDEX"+|For all tracks except the lead-out track, one or |
| |more track index points. |
+-----------------------+-------------------------------------------------+
Table 13
11.15. CUESHEET_TRACK_INDEX
+========+=========================================================+
| Data | Description |
+========+=========================================================+
| u(64) | Offset in samples, relative to the track offset, of the |
| | index point. For CD-DA, the offset MUST be evenly |
| | divisible by 588 samples (588 samples = 44100 samples/s |
| | * 1/75 s). Note that the offset is from the beginning |
| | of the track, not the beginning of the audio data. |
+--------+---------------------------------------------------------+
| u(8) | The index point number. For CD-DA, an index number of |
| | 0 corresponds to the track pre-gap. The first index in |
| | a track MUST have a number of 0 or 1, and subsequently, |
| | index numbers MUST increase by 1. Index numbers MUST |
| | be unique within a track. |
+--------+---------------------------------------------------------+
| u(3*8) | Reserved. All bits MUST be set to zero. |
+--------+---------------------------------------------------------+
Table 14
11.16. METADATA_BLOCK_PICTURE
+========+==================================================+
| Data | Description |
+========+==================================================+
| u(32) | The PICTURE_TYPE according to the ID3v2 APIC |
| | frame. |
+--------+--------------------------------------------------+
| u(32) | The length of the MIME type string in bytes. |
+--------+--------------------------------------------------+
| u(n*8) | The MIME type string, in printable ASCII |
| | characters 0x20-0x7E. The MIME type MAY also be |
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| | "-->" to signify that the data part is a URL of |
| | the picture instead of the picture data itself. |
+--------+--------------------------------------------------+
| u(32) | The length of the description string in bytes. |
+--------+--------------------------------------------------+
| u(n*8) | The description of the picture, in UTF-8. |
+--------+--------------------------------------------------+
| u(32) | The width of the picture in pixels. |
+--------+--------------------------------------------------+
| u(32) | The height of the picture in pixels. |
+--------+--------------------------------------------------+
| u(32) | The color depth of the picture in bits-per- |
| | pixel. |
+--------+--------------------------------------------------+
| u(32) | For indexed-color pictures (e.g. GIF), the |
| | number of colors used, or "0" for non-indexed |
| | pictures. |
+--------+--------------------------------------------------+
| u(32) | The length of the picture data in bytes. |
+--------+--------------------------------------------------+
| u(n*8) | The binary picture data. |
+--------+--------------------------------------------------+
Table 15
11.17. PICTURE_TYPE
+=======+=====================================+
| Value | Description |
+=======+=====================================+
| 0 | Other |
+-------+-------------------------------------+
| 1 | 32x32 pixels 'file icon' (PNG only) |
+-------+-------------------------------------+
| 2 | Other file icon |
+-------+-------------------------------------+
| 3 | Cover (front) |
+-------+-------------------------------------+
| 4 | Cover (back) |
+-------+-------------------------------------+
| 5 | Leaflet page |
+-------+-------------------------------------+
| 6 | Media (e.g. label side of CD) |
+-------+-------------------------------------+
| 7 | Lead artist/lead performer/soloist |
+-------+-------------------------------------+
| 8 | Artist/performer |
+-------+-------------------------------------+
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| 9 | Conductor |
+-------+-------------------------------------+
| 10 | Band/Orchestra |
+-------+-------------------------------------+
| 11 | Composer |
+-------+-------------------------------------+
| 12 | Lyricist/text writer |
+-------+-------------------------------------+
| 13 | Recording Location |
+-------+-------------------------------------+
| 14 | During recording |
+-------+-------------------------------------+
| 15 | During performance |
+-------+-------------------------------------+
| 16 | Movie/video screen capture |
+-------+-------------------------------------+
| 17 | A bright colored fish |
+-------+-------------------------------------+
| 18 | Illustration |
+-------+-------------------------------------+
| 19 | Band/artist logotype |
+-------+-------------------------------------+
| 20 | Publisher/Studio logotype |
+-------+-------------------------------------+
Table 16
Other values are reserved and SHOULD NOT be used. There MAY only be
one each of picture type 1 and 2 in a file.
11.18. FRAME
+==============+=================================+
| Data | Description |
+==============+=================================+
| FRAME_HEADER | |
+--------------+---------------------------------+
| "SUBFRAME"+ | One SUBFRAME per channel. |
+--------------+---------------------------------+
| u(?) | Zero-padding to byte alignment. |
+--------------+---------------------------------+
| FRAME_FOOTER | |
+--------------+---------------------------------+
Table 17
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11.19. FRAME_HEADER
+=======+================================+
| Data | Description |
+=======+================================+
| u(14) | Sync code '0b11111111111110' |
+-------+--------------------------------+
| u(1) | FRAME HEADER RESERVED |
+-------+--------------------------------+
| u(1) | BLOCKING STRATEGY |
+-------+--------------------------------+
| u(4) | INTERCHANNEL SAMPLE BLOCK SIZE |
+-------+--------------------------------+
| u(4) | SAMPLE RATE |
+-------+--------------------------------+
| u(4) | CHANNEL ASSIGNMENT |
+-------+--------------------------------+
| u(3) | SAMPLE SIZE |
+-------+--------------------------------+
| u(1) | FRAME HEADER RESERVED2 |
+-------+--------------------------------+
| u(?) | CODED NUMBER |
+-------+--------------------------------+
| u(?) | BLOCK SIZE INT |
+-------+--------------------------------+
| u(?) | SAMPLE RATE INT |
+-------+--------------------------------+
| u(8) | FRAME CRC |
+-------+--------------------------------+
Table 18
11.19.1. FRAME HEADER RESERVED
+=======+=========================+
| Value | Description |
+=======+=========================+
| 0 | mandatory value |
+-------+-------------------------+
| 1 | reserved for future use |
+-------+-------------------------+
Table 19
FRAME HEADER RESERVED MUST remain reserved for "0" in order for a
FLAC frame's initial 15 bits to be distinguishable from the start of
an MPEG audio frame (see also (http://lists.xiph.org/pipermail/flac-
dev/2008-December/002607.html)).
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11.19.2. BLOCKING STRATEGY
+=======+==================================+
| Value | Description |
+=======+==================================+
| 0 | fixed-blocksize stream; frame |
| | header encodes the frame number |
+-------+----------------------------------+
| 1 | variable-blocksize stream; frame |
| | header encodes the sample number |
+-------+----------------------------------+
Table 20
The "BLOCKING STRATEGY" bit MUST be the same throughout the entire
stream.
The "BLOCKING STRATEGY" bit determines how to calculate the sample
number of the first sample in the frame. If the bit is "0" (fixed-
blocksize), the frame header encodes the frame number as above, and
the frame's starting sample number will be the frame number times the
blocksize. If it is "1" (variable-blocksize), the frame header
encodes the frame's starting sample number itself. (In the case of a
fixed-blocksize stream, only the last block MAY be shorter than the
stream blocksize; its starting sample number will be calculated as
the frame number times the previous frame's blocksize, or zero if it
is the first frame).
11.19.3. INTERCHANNEL SAMPLE BLOCK SIZE
+=================+=========================================+
| Value | Description |
+=================+=========================================+
| 0b0000 | reserved |
+-----------------+-----------------------------------------+
| 0b0001 | 192 samples |
+-----------------+-----------------------------------------+
| 0b0010 - 0b0101 | 576 * (2^(n-2)) samples, i.e. 576, |
| | 1152, 2304 or 4608 |
+-----------------+-----------------------------------------+
| 0b0110 | get 8 bit (blocksize-1) from end of |
| | header |
+-----------------+-----------------------------------------+
| 0b0111 | get 16 bit (blocksize-1) from end of |
| | header |
+-----------------+-----------------------------------------+
| 0b1000 - 0b1111 | 256 * (2^(n-8)) samples, i.e. 256, 512, |
| | 1024, 2048, 4096, 8192, 16384 or 32768 |
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+-----------------+-----------------------------------------+
Table 21
11.19.4. SAMPLE RATE
+========+=====================================================+
| Value | Description |
+========+=====================================================+
| 0b0000 | get from STREAMINFO metadata block |
+--------+-----------------------------------------------------+
| 0b0001 | 88.2 kHz |
+--------+-----------------------------------------------------+
| 0b0010 | 176.4 kHz |
+--------+-----------------------------------------------------+
| 0b0011 | 192 kHz |
+--------+-----------------------------------------------------+
| 0b0100 | 8 kHz |
+--------+-----------------------------------------------------+
| 0b0101 | 16 kHz |
+--------+-----------------------------------------------------+
| 0b0110 | 22.05 kHz |
+--------+-----------------------------------------------------+
| 0b0111 | 24 kHz |
+--------+-----------------------------------------------------+
| 0b1000 | 32 kHz |
+--------+-----------------------------------------------------+
| 0b1001 | 44.1 kHz |
+--------+-----------------------------------------------------+
| 0b1010 | 48 kHz |
+--------+-----------------------------------------------------+
| 0b1011 | 96 kHz |
+--------+-----------------------------------------------------+
| 0b1100 | get 8 bit sample rate (in kHz) from end of header |
+--------+-----------------------------------------------------+
| 0b1101 | get 16 bit sample rate (in Hz) from end of header |
+--------+-----------------------------------------------------+
| 0b1110 | get 16 bit sample rate (in daHz) from end of header |
+--------+-----------------------------------------------------+
| 0b1111 | invalid, to prevent sync-fooling string of 1s |
+--------+-----------------------------------------------------+
Table 22
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11.19.5. CHANNEL ASSIGNMENT
For values 0b0000-0b0111, the value represents the (number of
independent channels)-1. Where defined, the channel order follows
SMPTE/ITU-R recommendations.
+==========+======================================================+
| Value | Description |
+==========+======================================================+
| 0b0000 | 1 channel: mono |
+----------+------------------------------------------------------+
| 0b0001 | 2 channels: left, right |
+----------+------------------------------------------------------+
| 0b0010 | 3 channels: left, right, center |
+----------+------------------------------------------------------+
| 0b0011 | 4 channels: front left, front right, back left, back |
| | right |
+----------+------------------------------------------------------+
| 0b0100 | 5 channels: front left, front right, front center, |
| | back/surround left, back/surround right |
+----------+------------------------------------------------------+
| 0b0101 | 6 channels: front left, front right, front center, |
| | LFE, back/surround left, back/surround right |
+----------+------------------------------------------------------+
| 0b0110 | 7 channels: front left, front right, front center, |
| | LFE, back center, side left, side right |
+----------+------------------------------------------------------+
| 0b0111 | 8 channels: front left, front right, front center, |
| | LFE, back left, back right, side left, side right |
+----------+------------------------------------------------------+
| 0b1000 | left/side stereo: channel 0 is the left channel, |
| | channel 1 is the side(difference) channel |
+----------+------------------------------------------------------+
| 0b1001 | right/side stereo: channel 0 is the side(difference) |
| | channel, channel 1 is the right channel |
+----------+------------------------------------------------------+
| 0b1010 | mid/side stereo: channel 0 is the mid(average) |
| | channel, channel 1 is the side(difference) channel |
+----------+------------------------------------------------------+
| 0b1011 - | reserved |
| 0b1111 | |
+----------+------------------------------------------------------+
Table 23
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11.19.6. SAMPLE SIZE
+=======+====================================+
| Value | Description |
+=======+====================================+
| 0b000 | get from STREAMINFO metadata block |
+-------+------------------------------------+
| 0b001 | 8 bits per sample |
+-------+------------------------------------+
| 0b010 | 12 bits per sample |
+-------+------------------------------------+
| 0b011 | reserved |
+-------+------------------------------------+
| 0b100 | 16 bits per sample |
+-------+------------------------------------+
| 0b101 | 20 bits per sample |
+-------+------------------------------------+
| 0b110 | 24 bits per sample |
+-------+------------------------------------+
| 0b111 | reserved |
+-------+------------------------------------+
Table 24
For subframes that encode a difference channel, the sample size is
one bit larger than the sample size of the frame, in order to be able
to encode the difference between extreme values.
11.19.7. FRAME HEADER RESERVED2
+=======+=========================+
| Value | Description |
+=======+=========================+
| 0 | mandatory value |
+-------+-------------------------+
| 1 | reserved for future use |
+-------+-------------------------+
Table 25
11.19.8. CODED NUMBER
Frame/Sample numbers are encoded using the UTF-8 format, from BEFORE
it was limited to 4 bytes by RFC3629, this variant supports the
original 7 byte maximum.
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Note to implementors: All Unicode compliant UTF-8 decoders and
encoders are limited to 4 bytes, it's best to just write your own one
off solution.
if(variable blocksize)
`u(8...56)`: "UTF-8" coded sample number (decoded number is 36 bits)
else
`u(8...48)`: "UTF-8" coded frame number (decoded number is 31 bits)
11.19.9. BLOCK SIZE INT
if(`INTERCHANNEL SAMPLE BLOCK SIZE` == 0b0110)
8 bit (blocksize-1)
else if(`INTERCHANNEL SAMPLE BLOCK SIZE` == 0b0111)
16 bit (blocksize-1)
11.19.10. SAMPLE RATE INT
if(`SAMPLE RATE` == 0b1100)
8 bit sample rate (in kHz)
else if(`SAMPLE RATE` == 0b1101)
16 bit sample rate (in Hz)
else if(`SAMPLE RATE` == 0b1110)
16 bit sample rate (in daHz)
11.19.11. FRAME CRC
CRC-8 (polynomial = x^8 + x^2 + x^1 + x^0, initialized with 0) of
everything before the CRC, including the sync code
11.20. FRAME_FOOTER
+=======+===================================================+
| Data | Description |
+=======+===================================================+
| u(16) | CRC-16 (polynomial = x^16 + x^15 + x^2 + x^0, |
| | initialized with 0) of everything before the CRC, |
| | back to and including the frame header sync code |
+-------+---------------------------------------------------+
Table 26
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11.21. SUBFRAME
+============================================+======================+
| Data | Description |
+============================================+======================+
| SUBFRAME_HEADER | |
+--------------------------------------------+----------------------+
| "SUBFRAME_CONSTANT" || "SUBFRAME_FIXED" || | The SUBFRAME_HEADER |
| "SUBFRAME_LPC" || "SUBFRAME_VERBATIM" | specifies which |
| | one. |
+--------------------------------------------+----------------------+
Table 27
11.22. SUBFRAME_HEADER
+========+========================================================+
| Data | Description |
+========+========================================================+
| u(1) | Zero bit padding, to prevent sync-fooling string of 1s |
+--------+--------------------------------------------------------+
| u(6) | "SUBFRAME TYPE" (see section on SUBFRAME TYPE |
| | (#subframe-type)) |
+--------+--------------------------------------------------------+
| u(1+k) | "WASTED BITS PER SAMPLE FLAG" (see section on WASTED |
| | BITS PER SAMPLE FLAG (#wasted-bits-per-sample-flag)) |
+--------+--------------------------------------------------------+
Table 28
11.22.1. SUBFRAME TYPE
+==========+================================+
| Value | Description |
+==========+================================+
| 0b000000 | SUBFRAME_CONSTANT |
+----------+--------------------------------+
| 0b000001 | SUBFRAME_VERBATIM |
+----------+--------------------------------+
| 0b00001x | reserved |
+----------+--------------------------------+
| 0b0001xx | reserved |
+----------+--------------------------------+
| 0b001xxx | if(xxx <= 4) "SUBFRAME_FIXED", |
| | xxx=order; else reserved |
+----------+--------------------------------+
| 0b01xxxx | reserved |
+----------+--------------------------------+
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| 0b1xxxxx | "SUBFRAME_LPC", xxxxx=order-1 |
+----------+--------------------------------+
Table 29
11.22.2. WASTED BITS PER SAMPLE FLAG
+=======+==============================================+
| Value | Description |
+=======+==============================================+
| 0 | no wasted bits-per-sample in source |
| | subblock, k=0 |
+-------+----------------------------------------------+
| 1 | k wasted bits-per-sample in source subblock, |
| | k-1 follows, unary coded; e.g. k=3 => 0b001 |
| | follows, k=7 => 0b0000001 follows. |
+-------+----------------------------------------------+
Table 30
The size of the samples stored in the subframe is the subframe sample
size reduced by k bits. Decoded samples must be shifted left by k
bits.
11.23. SUBFRAME_CONSTANT
+======+========================================+
| Data | Description |
+======+========================================+
| u(n) | Unencoded constant value of the |
| | subblock, n = frame's bits-per-sample. |
+------+----------------------------------------+
Table 31
11.24. SUBFRAME_FIXED
+==========+========================================+
| Data | Description |
+==========+========================================+
| u(n) | Unencoded warm-up samples (n = frame's |
| | bits-per-sample * predictor order). |
+----------+----------------------------------------+
| RESIDUAL | Encoded residual |
+----------+----------------------------------------+
Table 32
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11.25. SUBFRAME_LPC
+==========+========================================================+
| Data | Description |
+==========+========================================================+
| u(n) | Unencoded warm-up samples (n = frame's bits- |
| | per-sample * lpc order). |
+----------+--------------------------------------------------------+
| u(4) | (quantized linear predictor coefficients' |
| | precision in bits)-1 (NOTE: 0b1111 is invalid). |
+----------+--------------------------------------------------------+
| u(5) | Quantized linear predictor coefficient shift |
| | needed in bits (NOTE: this number is signed |
| | two's-complement). |
+----------+--------------------------------------------------------+
| u(n) | Unencoded predictor coefficients (n = qlp coeff |
| | precision * lpc order) (NOTE: the coefficients |
| | are signed two's-complement). |
+----------+--------------------------------------------------------+
| RESIDUAL | Encoded residual |
+----------+--------------------------------------------------------+
Table 33
11.26. SUBFRAME_VERBATIM
+=========+===============================================+
| Data | Description |
+=========+===============================================+
| u(n\*i) | Unencoded subblock, where "n" is frame's |
| | bits-per-sample and "i" is frame's blocksize. |
+---------+-----------------------------------------------+
Table 34
11.27. RESIDUAL
+==================================================+======================+
|Data |Description |
+==================================================+======================+
|u(2) |RESIDUAL_CODING_METHOD|
+--------------------------------------------------+----------------------+
|"RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB" ||| |
|"RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2" | |
+--------------------------------------------------+----------------------+
Table 35
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11.27.1. RESIDUAL_CODING_METHOD
+=======+========================================================+
| Value | Description |
+=======+========================================================+
| 0b00 | partitioned Exp-Golomb coding with 4-bit Exp-Golomb |
| | parameter; |
| | RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB follows |
+-------+--------------------------------------------------------+
| 0b01 | partitioned Exp-Golomb coding with 5-bit Exp-Golomb |
| | parameter; |
| | RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2 follows |
+-------+--------------------------------------------------------+
| 0b10 | reserved |
| - | |
| 0b11 | |
+-------+--------------------------------------------------------+
Table 36
11.27.2. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB
+=========================+===================================+
| Data | Description |
+=========================+===================================+
| u(4) | Partition order. |
+-------------------------+-----------------------------------+
| "EXP_GOLOMB_PARTITION"+ | There will be 2^order partitions. |
+-------------------------+-----------------------------------+
Table 37
11.27.2.1. EXP_GOLOMB_PARTITION
+==========+====================================================+
| Data | Description |
+==========+====================================================+
| u(4(+5)) | "EXP-GOLOMB PARTITION ENCODING PARAMETER" (see |
| | section on EXP-GOLOMB PARTITION ENCODING PARAMETER |
| | (#exp-golomb-partition-encoding-parameter)) |
+----------+----------------------------------------------------+
| u(?) | "ENCODED RESIDUAL" (see section on ENCODED |
| | RESIDUAL (#encoded-residual)) |
+----------+----------------------------------------------------+
Table 38
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11.27.2.2. EXP GOLOMB PARTITION ENCODING PARAMETER
+==========+==========================================+
| Value | Description |
+==========+==========================================+
| 0b0000 - | Exp-golomb parameter. |
| 0b1110 | |
+----------+------------------------------------------+
| 0b1111 | Escape code, meaning the partition is in |
| | unencoded binary form using n bits per |
| | sample; n follows as a 5-bit number. |
+----------+------------------------------------------+
Table 39
11.27.3. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2
+==========================+===================================+
| Data | Description |
+==========================+===================================+
| u(4) | Partition order. |
+--------------------------+-----------------------------------+
| "EXP-GOLOMB2_PARTITION"+ | There will be 2^order partitions. |
+--------------------------+-----------------------------------+
Table 40
11.27.3.1. EXP_GOLOMB2_PARTITION
+==========+=====================================================+
| Data | Description |
+==========+=====================================================+
| u(5(+5)) | "EXP-GOLOMB2 PARTITION ENCODING PARAMETER" (see |
| | section on EXP-GOLOMB2 PARTITION ENCODING PARAMETER |
| | (#expgolomb2-partition-encoding-parameter)) |
+----------+-----------------------------------------------------+
| u(?) | "ENCODED RESIDUAL" (see section on ENCODED RESIDUAL |
| | (#encoded-residual)) |
+----------+-----------------------------------------------------+
Table 41
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11.27.3.2. EXP-GOLOMB2 PARTITION ENCODING PARAMETER
+===========+==========================================+
| Value | Description |
+===========+==========================================+
| 0b00000 - | Exp-golomb parameter. |
| 0b11110 | |
+-----------+------------------------------------------+
| 0b11111 | Escape code, meaning the partition is in |
| | unencoded binary form using n bits per |
| | sample; n follows as a 5-bit number. |
+-----------+------------------------------------------+
Table 42
11.27.4. ENCODED RESIDUAL
The number of samples (n) in the partition is determined as follows:
* if the partition order is zero, n = frame's blocksize - predictor
order
* else if this is not the first partition of the subframe, n =
(frame's blocksize / (2^partition order))
* else n = (frame's blocksize / (2^partition order)) - predictor
order
12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Authors' Addresses
Michael Sandelman
Email: mcr@sandelman.ca
Andrew Weaver
Sandelman & Weaver Expires 29 October 2021 [Page 34]
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Email: theandrewjw@gmail.com
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